1.7 -> 1.8 is a 5.9% increase, which is roughly the increase given for T20 -> T21.
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The compendium is based on my work, simulations, and advice, so the information contained therein is perfectly valid. There's simply no reason to create a competing source of the same information on MMO-C, though you can always reference my WoWhead and Icy-Veins guides as well.